The present invention relates to catalysts and a novel process for the upgrading .alpha.-hydroxycarboxylic acids to more useful and valuable chemical compounds. In another aspect, the invention relates to a novel process for selectively converting lactic acids to useful products such as propanoic acid, acetic acid and acetaldehyde.
Catalyst systems similar to the one employed in this inventive process are used to decarboxylate alkyl aromatic compounds. For example, U.S. Pat. No. 4,319,066 teaches a process of preparing alkenyl aromatic compounds by employing a decarboxylation catalyst. U.S. Pat. No. 4,393,260 teaches a process for the preparation of cyclohexanyl compounds by employing a decarboxylation catalyst. These processes do not include .alpha.-hydroxycarboxylic acids substrates as are employed in this inventive process.
Lactic acid, a reactant of the inventive process, is a hydroxycarboxylic acid that is an inexpensive material that can be manufactured synthetically or by fermentation. Hydroxycarboxylic acids contain two functional groups, both a carboxy and a hydroxyl group. Lactic acid is a useful intermediate chemical compound that can be converted into more valuable industrial products such as propanoic acid, acetic acid, acetaldehyde, acrylic acid, ethanol, propanol, etc.
The inventive process converts .alpha.-hydroxycarboxylic acids to hydroxy free aliphatic acids and aldehydes. Methods for producing the products of the inventive process and in particular the manufacture of propanoic acid, acetic acid and acetaldehyde are described below.
The major method of manufacturing propanoic acid is by the oxo-process or hydrocarbon-oxidation process. The oxo-process is the reaction of ethylene and carbon monoxide under reductive conditions to produce propionaldehyde which is further oxidized to the acid. The hydrocarbon-oxidation route is the liquid-phase oxidation of propane, butane, mixed paraffins and paraffin wax to the acid. A further process is the direct oxidation of 1-propanol with nitric acid.
The methods of producing acetic acid have shifted from natural fermentation processes to synthetic processes. Natural fermentation processes include alcohol fermentation and destructive distillation of hard wood. Principle synthetic processes employed include oxidation of acetaldehyde, direct oxidation of ethanol, hydrocarbon-oxidation and methanol-carbon monoxide process.
The major method of producing acetaldehyde is by liquid-phase oxidation of ethylene. Further commercial processes are by partial oxidation of ethyl alcohol and the hydration of acetylene. Acetaldehyde is also formed as a coproduct in the vapor phase oxidation of saturated hydrocarbons. A more recent process for producing acetaldehyde is by the direct conversion of synthesis gas in the presence of a rhodium catalyst.
For various reasons, none of the above processes are entirely satisfactory. Further, propanoic acid, acetic acid and acetaldehyde are not typically produced from lactic acid. Consequently, there exists an interest in developing new processes for upgrading lactic acid to more valuable chemical compounds. Thus, it is the object of this invention to provide a novel process for the conversion of .alpha.-hydroxycarboxylic acids to selectively form aliphatic acids, aldehydes, ketones and alcohols. It is another object of this invention to produce valuable chemicals, such as propanoic acid, acetic acid and acetaldehyde.
This and other objects of the invention will become apparent in the description of the invention and the examples which follow.